1. Field of the Invention
The present invention concerns a subsea separation apparatus for treating crude oil comprising a separator module with a separator tank for the separation of water, gas, sand and crude oil from fluids flowing from a well. The apparatus or plant is designed to be placed on the seabed adjacent to, or on the well.
2. Description of Related Art
During the exploitation of oil and gas offshore, the produced fluids are usually led to a plant, for instance on an offshore platform, for the separation of the various phases of the fluid. These phases mainly include hydrocarbons, water, and in some cases, sand. The separation is usually performed with conventional separation equipment such as hydrocyclones and sedimentation tanks.
However, there are disadvantages connected to placing such separation equipment on offshore platforms. In many cases, the platforms impose limitations upon the weight and other design parameters of such plants. Risers and other equipment for bringing the well fluids to the platform and plant must be dimensioned to lead larger amounts of fluid to the platform than are to be produced. In addition, any reinjection of water into the well will require a lot of additional equipment for leading separated water down into the well. Alternatively, cleaned, but still oil containing water may be dumped directly into the sea, but this is problematic in areas sensitive to pollution.
It has therefore been suggested to place the separation plant on the seabed. This reduces the requirement for surface placed plants, and the need to transport fluids to the surface is reduced by the same amount as the water fraction in the fluid.
Such subsea plants known. For instance, Norwegian Patent NO 304388 which describes a method and an apparatus for the separation of a hydrocarbon flow on the seabed. The patent describes a subsea plant for the separation of crude oil fractions. The solution primarily concerns an arrangement and a method for sand processing, and appears limited to the separation of sand in produced water. The processing of sand takes place in the arrangement after pressurization. The sand is taken out downstream of the separator, where the water has a lower pressure. A water injection pump is used to increase the pressure to a pressure just over the pressure in the separator tank to force the sand to flow into the oil and gas export line. The disadvantage of this is that the pump must circulate sand containing water, which involves a considerable risk for increased wear with correspondingly high maintenance frequency. This is very cost intensive.
The handling of sand production is a common problem for underwater separation plants. Norwegian patent NO B1 172555 describes an underwater station for the handling and transport of a well flow. The underwater station is placed on the seabed and is intended to separate crude oil fractions in a manner similar to the present invention. In particular, a multi-phase flow is transformed to two phases, a gas phase and a liquid phase, such that the flow as a whole can be moved by means of a single phase pump and a gas compressor.
Hence, the separation is primarily motivated by the transporting needs, meaning that a known single phase compression technology for pressurization allows the transfer of the well flow over greater distances than in the case of a pressure drop driven transport. It appears that the pump is mounted directly under the separator, and the compressor is mounted directly over the separator, in a vertical assembly. However, the present invention includes a three-phase separation where gas, oil, and water are treated with the purpose of removing the water fraction.
Norwegian Patent NO B1 309587 shows an apparatus for the separation of an oil/liquid phase from a gas phase in a well head fluid. The patent describes an arrangement for centrifugal separation that either can be placed at the surface or as a part of a subsea plant, for two-phase separation of crude oil or wet gas.
The apparatus is intended to inflict gravitational forces on the well fluid beyond normal gravitation (g), with the purpose of being able to part the liquid phase from the gas phase quickly, that is two-phase separation.